Analog inductive transmission with a transmit and receive coil in the baseband is predominantly used for the purpose of wireless data transmission to hearing devices for telephoning or for inductive reception in buildings, e.g. churches. Here a large transmit coil is generally used in the floor or in the telephone and a correspondingly aligned receive coil is used in the hearing device.
In principle, in the case of inductive audio transmission by means of transmit and receive coils, the problem consists in that a significantly reduced signal quality can be expected if the coils are not aligned optimally in relation to one another. To this end, the left side of FIG. 1 shows an optimal alignment of a transmit coil SS and a receive coil SE. The two coils SS and SE are arranged coaxially here so that a transmission with the highest degree of efficiency results, since the receive coil SE lies parallel to the magnetic field lines.
If on the other hand the receive coil SE is not parallel to the magnetic field lines, the degree of efficiency of the inductive transmission drops accordingly. The most minimal receive signal can then be expected if the axis of the receive coil SE is perpendicular to the magnetic field lines. This case is shown on the right in FIG. 1.
The problem of inadequate alignment of the transmit and receive coils is intensified in that further to the development of hearing devices with digital, inductive receivers, the telephone coil is increasingly housed in a hearing device remote controller. Such a remote controller can and is essentially more easily moved than a hearing device. It will thus also frequently assume an inadequate alignment in respect of the magnetic field of the transmit coil.
Methods are known from the publication DE 201 14 461, with which the problem of reduced signal quality in the case of poor alignment can be counteracted by using a number of coils. A number of coils are positioned orthogonal to one another and search out the receive signal, which supplies the strongest useful signal, from the receive signals. If necessary, the input signals are weighted differently. However optimum reception is not always achieved by this means, irrespective of the alignment of the transmit and receive coils.
The publication DE 601 09 268 T2 discloses an apparatus for controlling an antenna. This comprises a triaxial magnetic bearing sensor having two magnetic bearing sensors, each of which detects two components of the geomagnetic vector in the directions of two of the three axes of a right-hand coordinate system. Each of the two magnetic bearing sensors measures voltages which are excited in two coils arranged orthogonal to one another.
Furthermore, the publication DE 38 54 051 T2 discloses an antenna structure for generating a uniform field. The antennas have coils, which are arranged according to orthogonal x-, y- and z-axes.